A key tool in understanding how variables are related is correlation, or the tendency of one variable to increase or decrease as another variable changes. Students can explore positive and negative correlation in the Correlation Gizmo. In the Gizmo, students can adjust the r-value of a data set and estimate least-squares best fit lines for the data.

Of course, it is always important to remember that correlation does not imply causation. For example, the past decade has seen a rise in sunscreen usage and Katy Perry album sales. However, this does not mean that using sunscreen makes you want to listen to Katy Perry music. (Or does it?)